This invention relates to apparatus for controlling the injection timing of a fuel injection pump in a diesel engine. Such mechanisms are well known wherein the engine is provided with a pump which supplies fluid pressure proportional to engine speed. The fluid pressure acts on a piston in a cylinder in opposition to a return spring and moves the piston to advance injection timing at higher engine speeds.
Conventional injection timing devices provide proper control of injection timing when the engine is at its normal operating temperature, and provide advancement of the timing with increased engine speed. Since the fluid pressure supplied to the device is proportional to engine speed, and the force of the return spring on the piston is proportional to displacement, a linear variation of injection timing is obtained over the full engine speed operating range with the result of improved emission control.
During cold starting and warming up of the engine, it is desirable to have a more advanced timing of injection, particularly at low speed, to achieve a favorable combustion condition and a reduction of the "blue smoke period".
Published German Patent Application No. 1,143,675 discloses a device which includes inner and outer injection timing adjustment pistons, both of which are acted upon by the speed proportional fluid pressure in opposition to respective inner and outer return springs. The return spring of the inner piston is weaker than that of the outer piston so that the inner piston becomes fully displaced at a relatively low supplied pressure. The outer piston has a stronger spring and is arranged for displacement in the direction opposite to that of the inner piston. The inner piston is connected to the injection timing device of the fuel injection pump. Prior to initial starting of the engine, under the influence of the inner and outer return springs, the pistons cause the fuel pump to have an advanced injection timing condition. After the engine has been started and a relatively low speed proportional fluid pressure is supplied to the pump, the inner piston is rapidly displaced against its weaker spring to a retarded injection timing condition. Thereafter, the inner and outer pistons move together in a direction opposite to the original movement of the inner piston under the influence of the supplied fluid pressure to advance injection timing in proportion to supplied pressure and consequently engine speed.
This prior art injection control device provides an injection timing advance for initial engine starting, but immediately upon engine starting, upon displacement of the inner piston, the injection timing is retarded into the normal injection timing condition, independent of the operating temperature of the engine. Therefore, while there is provided advanced injection timing during engine starting, there is no advanced injection timing during the warm-up period. Further, this prior mechanism with dual pistons is relatively expensive to construct.
Recognizing that different injection advancement conditions are required for favorable engine operation during the warm-up period and normal temperature engine operation, and that a timing condition which is favorable for operating temperature is unfavorable for a cold engine, the adjustment of the injection timing merely in accordance with the amount of supplied speed proportional pressure, as in this prior device, does not provide sufficient control of injection timing for smooth and pollution-free engine operation particularly at low engine speeds.
Another prior adjustment device for fuel injection timing is disclosed in German Published Patent Application No. 1,094,527. The device therein disclosed includes a valve in the conduit supplying speed proportional pressure to the timing adjustment cylinder. The valve is designed to open only upon the attainment of a predetermined minimum pressure. Accordingly, the operation of the fuel injection timing adjustment device is also controlled only in accordance with speed proportional pressure, and not in accordance with engine temperature, as is required for optimum engine operation.
It is therefore an object of the invention to provide an adjustment device for a fuel injection pump in a diesel engine which is relatively inexpensive to construct, and which provides injection timing which is adjusted for the temperature condition of the internal combustion engine as well as operating engine speed.